Lease breaks happen. Landlords hate them and often get really grumpy when their tenants don't honor the full term of their lease agreements. They're totally justified in being dour about it, especially if the tenant takes off in the middle of the winter. It's an inconvenience, a hit to the wallet, a bad time for a vacancy, not to mention a broken promise. However, it's important to remember that tenants, especially with home prices as low as they are here in late 2012, are renting for a reason. They know they can't commit to a long-term home ownership situation. They know they may have to cut out early. They are renting because they are trying to be responsible about their large-scale purchases. The whole point of renting is that it allows for a much easier "out."
I'd say that about 30-40% of the 1200+ leases that I have worked on since 2005 have wound up with the tenant leaving in the middle of a contract term. They may stay for a year, renew the lease, and then leave halfway through the second year, they may bolt in the middle of their first month, or they may even stay for 10 years and then suddenly have to depart. No matter what, it's still means a broken lease and the potential for some very bad blood between the landlord and their departing tenant.
In Chicago, there's two standard routes that a responsible tenant can take to break a lease. One is subleasing and the other is rerenting. They are often confused and somewhat murky. So, in the interest of making everything very clear, I've gone ahead and made a twee pink infographic for you, available after the jump.